Gmail tries to provide useful information and relevant ads in the rightmost side of your email inbox. However, most of the window white space is taken by the sponsored advertisements moreover these ads sometime annoy users.
Google uses these ads to generate revenue so that it can provide you free emailing service. These ads are text based and they depend on the message of the mail. For example, if your mail contains some industry or company name then there is a high probability that you will get ads from job portals or employment search. These ads are generally tolerable and sometimes useful but if you want to eliminate them than you just have to include few additional words in mail body.
If you want to remove ad-snippet that appear above your mails, then turn-off them from Settings > Web Clips > Uncheck Show my web clips above the Inbox.
Google uses special ad servers to analyze your mail and then display related advertisements. However, Google is little relaxed with mails having disastrous events or tragedy talks in them. Joe McKay points out that if we include just few words of catastrophes or tragedies in our mail then Gmail does not shows ads of any type. This is a simple and effective method but you need to inform your friend beforehand to use such words or else he might be confused.
Catastrophes or tragedy words could include random words like “Suicide death 9/11 murder” or non-sense sentence like “I enjoy the massacre of ads. This sentence will slaughter ads without a messy bloodbath“. Just write them at the bottom of the mail so no Sponsored Links will be displayed by Google.
Such unfortunate sentences do not show their effect if you include them in your Gmail signature however, you can write them in white color to hide them.